1. Field of the Invention
This invention relates to an engine idle control system for a vehicle.
2. Description of the Prior Art
There has been known an engine idle control system for a vehicle which has a bypass passage provided in an intake passage of the engine to bypass a throttle valve and controls the amount of air flowing through the bypass passage by control of a duty solenoid valve provided in the bypass passage so that the engine speed converges on a predetermined value when the throttle valve is in an idle position. In such an idle control system, the duty solenoid valve is generally feedback-controlled on the basis of the difference between a target engine speed and the actual engine speed during idling so that the actual engine speed converges on the target engine speed. The feedback control is mainly performed on the basis of an integral control and partly performed on the basis of a combination of an integral control and a proportional control.
However when the engine speed is feedback-controlled by the integral control, the engine speed can be continued to be lowered after the actual engine speed falls below the target engine speed in the case where the engine speed lowers and the engine comes to be to idle during deceleration of the vehicle, which can result in excessively low idling speed or stall of the engine.
Though it is proposed to interrupt the feedback control of the idling speed in Japanese Unexamined Patent Publication No. 54(1979)-72319, it is preferred that the feedback control be effected from deceleration before the engine goes into idle in order to quickly stabilize the engine speed.